multiqueue: post 100% buffering if single queue is not linked

This makes buffering stop in case a stream switch happens. This is
important for adaptive streams that can disable not-linked streams
to avoid consuming the network bandwidth.

https://bugzilla.gnome.org/show_bug.cgi?id=719575
funnel: Proxy CAPS and ALLOCATION queries
funnel: Setting the PROXY_CAPS flag on the srcpad does not make much sense

funnel outputs whatever one of the upstreams currently outputs, a caps
query to a random upstream does not give the right answer here.